In recent years, problems such as global warming by CO2 emission and deletion of energy resources are becoming increasingly serious, and as one of countermeasures thereto, photovoltaic generation using energy from the inexhaustibly shining sunlight is drawing attention. The photovoltaic generation is a power generation system in which the energy from sunlight is directly converted to electric power by use of a solar battery, and a polycrystalline silicon wafer is predominantly used for a substrate for the solar battery.
A polycrystalline silicon wafer for a solar battery uses a unidirectionally solidified silicon ingot as starting material, and is produced by slicing the ingot. Therefore, in order to get the solar batteries to be in widespread use, the cost of the silicon wafer has to be reduced while securing the quality thereof. Further, as a stage prior to the production of the silicon wafer by slicing, producing a high-quality silicon ingot with low cost is demanded.
In the quality demand for polycrystalline silicon ingot, the concentration of impurities contained in the silicon ingot is particularly important. This is because contamination of a polycrystalline silicon wafer, as being cut from the silicon ingot, with impurities such as Mo, Fe and Cu causes, when applied to a solar battery, deterioration of photoelectric conversion efficiency. For example, a relationship between impurity concentration and photoelectric conversion efficiency in a polycrystalline silicon wafer is shown in Non-Patent Literature 1.
FIG. 2 is a view showing a relationship between impurity concentration and relative conversion efficiency in a polycrystalline silicon wafer. FIG. 2 is a citation from FIG. 3 given in Non-Patent Literature 1, p. 532. FIG. 2 indicates a relationship between concentrations of Mo, Fe, Cu, C, O and Al, which are impurities, and relative conversion efficiency in a solar battery using a P-type polycrystalline silicon wafer. It can be confirmed from FIG. 2 that the relative conversion efficiency is deteriorated when the concentration of each impurity exceeds a specific concentration.
In Non-Patent Literature 2, a relationship between impurity concentration and photoelectric conversion efficiency is shown although it relates to a monocrystalline silicon wafer for a solar battery.
FIG. 3 is a view showing a relationship between impurity concentration and relative conversion efficiency in a monocrystalline silicon wafer. FIG. 3 is a citation from FIG. 2-4-12 given in Non-Patent Literature 2, p. 105. FIG. 3 indicates a relationship between concentrations of Ta, Mo, Nd, Zr, W, Ti, V, Cr, Mn, Fe, Co, Al, Ni, Cu and P, which are impurities, and relative conversion efficiency in a solar battery using a P-type monocrystalline silicon wafer. It can be confirmed from FIG. 3 that the relative conversion efficiency is deteriorated when the concentration of each impurity exceeds a specific concentration.
Therefore, it is recommended for a polycrystalline silicon wafer for a solar battery to reduce the contamination with impurities as much as possible. Thus, it is demanded for a silicon ingot that is used as the starting material of the polycrystalline silicon wafer to reduce the contamination with impurities as much as possible.
As a method capable of responding to such demands for cost and quality, an EMC (Electromagnetic Casting) process that is a continuous casting method using electromagnetic induction is put into practical use.
FIG. 4 is a schematic view showing a configuration of a continuous casting apparatus (hereinafter simply referred also to as “EMC furnace”) which is used in the conventional EMC process. As shown in FIG. 4, the EMC furnace is provided with a chamber 1. The chamber 1 is a water-cooled container having a double-walled structure, which is configured so that the inside is isolated from the outside air and maintained in an inert gas atmosphere suitable for casting. A raw material supply device, not shown, is connected to the upper wall of the chamber 1 through an openable and closable shutter 2. The chamber 1 includes an inert gas inlet 5 provided at an upper portion of a side wall and an outlet 6 provided at a lower portion of the side wall.
A bottomless cold crucible 7, an induction coil 8 and an after-heater 9 are disposed inside the chamber 1. The bottomless cold crucible 7 functions not only as a melting container but also as a casting mold. The cold crucible 7 is composed of a square cylinder made of metal excellent in heat conductivity and electric conductivity (e.g., copper), and suspended within the chamber 1. The cold crucible 7 has a part along an axial direction that is circumferentially divided into a plurality of sections as being strip elements. The cold crucible 7 is forcedly cooled by cooling water circulated inside.
The induction coil 8 is provided concentrically with the cold crucible 7 so as to surround the cold crucible 7, and is connected to a power-supply unit, not shown. A plurality of after-heaters 9 are continuously provided below the cold crucible 7 in a concentric manner with the cold crucible 7. An ingot 3 being pulled down from the cold crucible 7 is heated by the after-heater 9, and cooled to room temperature over time while causing an appropriate temperature gradient therein along an axial direction.
In the inside of the chamber 1, a raw material inlet tube 11 is attached below the shutter 2 connected to the raw material supply device. Granular and/or lump-like silicon raw materials 12 are supplied from the raw material supply device to the raw material inlet tube 11 in accordance with the opening and closing of the shutter 2, and charged into the cold crucible 7.
An outlet port 4 for pulling the ingot 3 below the after-heaters 9 is provided on the bottom wall of the chamber 1, and the outlet port 4 is sealed with gas. The ingot 3 is pulled down while being supported by a support stand 15 that can be lowered through the outlet port 4.
A plasma torch 14 is provided just above the cold crucible 7 so as to be movable up and down. The plasma torch 14 is connected to one of poles of a plasma power-supply unit, not shown, the other pole thereof being connected to the ingot 3 side. The plasma torch 14 is used in a state where it is lowered and inserted to the cold crucible 7.
In the EMC process using such an EMC furnace, the silicon raw materials 12 are charged in the bottomless cold crucible 7, and the lowered plasma torch 14 is electrified while applying an AC current to the induction coil 8. At that time, since the strip elements constituting the cold crucible 7 are electrically separated from each other, eddy current is generated within each element, accompanied by the electromagnetic induction by the induction coil 8, and the eddy current on the inner wall of the cold crucible 7 generates a magnetic field within the cold crucible 7. The silicon raw materials inside the cold crucible 7 are thus molten through electromagnetic induction heating to form molten silicon 13. Further, plasma arc is generated between the plasma torch 14 and the silicon raw materials and also between the plasma torch 14 and the molten silicon 13. The silicon raw materials are heated and molten also by the Joule heat of the plasma arc. The molten silicon 13 is thus efficiently formed while reducing the load of the electromagnetic induction heating.
The molten silicon 13 receives a force (pinching force, refer to solid arrows in FIG. 4) in the inward normal direction of the outer side surface 13a of the molten silicon by an interaction between the magnetic field associated with the eddy current on the inner wall 7a of the cold crucible and the current generated on the side surface of the molten silicon 13. Therefore, the inner surface 7a of the cold crucible and the outer side surface 13a of the molten silicon are kept in a non-contact state.
When the support stand 15 supporting the molten silicon 13 is gradually lowered while the silicon raw materials 12 being melted inside the bottomless cold crucible 7, the induction magnetic field becomes small with a distance from the lower end of the induction coil 8 increasing. Due to the resulting reduction in heat value and pinching force and the cooling from the cold crucible 7, solidification progresses from the outer side surface 13a of the molten silicon and the outer surface 3a of the ingot. The silicon raw materials 12 are continuously charged in association with the lowering of the support stand 15 to continue the melting and solidification, whereby the molten silicon 13 is solidified unidirectionally, and the ingot 3 can be thus continuously casted.
According to such an EMC process, the contact of the outer side surface 13a of the molten silicon with the inner surface 7a of the bottomless cold crucible is significantly reduced. Therefore, the contamination with impurities from the cold crucible 7 which is associated with this contact is mitigated, and a high-quality ingot 3 can be obtained. Furthermore, a unidirectionally-solidified ingot 3 can be inexpensively produced owing to the continuous casting.
However, in the continuous casting of silicon ingot by the EMC process, the gap between the cold crucible and the ingot is very small such that the dimension of cast ingot is substantially equal to the inner surface dimension of the cold crucible in a cross section perpendicular to the pull-down axis. Therefore, the outer surface of the ingot supported on the support stand can locally contact the inner surface of the cold crucible due to the accuracy of a lifting device for pulling down the support stand, causing adhesion of impurities. In this case, the adhered impurities diffuse from the outer surface of the ingot to the inside thereof, in the cooling step using the after-heater, and contaminate the ingot.
Further, when the ingot as being cast locally contacts the cold crucible, the inner surface of the cold crucible is damaged. This also causes reduction in the number of ingots that can be continuously cast with a single cold crucible, or reduction in life of the cold crucible.
With respect to this problem in which the local contact of the ingot as being cast with the cold crucible causes the damage to the cold crucible in addition to the contamination of the ingot with impurities, various proposals are made in the past, including, for example, Patent Literature 1. In Patent Literature 1, a continuous casting method of an ingot using a cold crucible with silicon coating being applied to the inner surface thereof is proposed. According to Patent Literature 1, the silicon coating on the inner surface of the cold crucible allows reduction in the damage to the cold crucible in addition to reduction in the contamination of the ingot with impurities.
However, during the continuous casting of a silicon ingot, the silicon coating applied to the inner surface of the cold crucible reduces the pinching force acting onto molten silicon and further reaches high temperature by the surface skin effect of induction heating. Therefore, the phenomenon in which the molten silicon locally contacts the silicon coating frequently occurs, and the silicon coating is, upon contact, fused to the molten silicon. As a result, the silicon coating applied to the inner surface of the cold crucible is apt to be partially peeled off.
When a silicon-coated cold crucible is used for continuous casting of a plurality of ingots, in the third cycle/operation onwards in continuous casting, the cast ingot can be contaminated with impurities due to the contact with the inner surface of the cold crucible which is exposed by the peel-off of the silicon coating. Therefore, further reduction in the contamination of the ingot with impurities from the cold crucible and also further reduction in the damage to the cold crucible are demanded for the conventional continuous casting method of the silicon ingot.